A copper foil has been used as a material for formation of wiring included in various types of electronic devices including printed wiring boards. In recent years, copper foils have been used not only as a material for formation of wiring but also as a negative electrode current collector of non-aqueous secondary batteries including lithium ion secondary battery.
A negative electrode material of a lithium ion secondary battery is provided with a negative electrode mixture layer including a negative electrode active substance, a conductive material, a binder on the surface of a negative electrode current collector made of a conductive material (for example, see “Patent Document 1”). If a negative electrode active substance store/release lithium in charging/discharging of lithium ion secondary battery, the volume of the negative electrode active substance changes. Expansion/contraction of the negative electrode mixture layer applies a stress between the negative electrode mixture layer and the current collector since the negative electrode mixture layer adheres to the surface of a negative electrode current collector. Repeated charging/discharging cycle applies stresses on the negative electrode current collector again and again. If tensile strength of the negative electrode current collector is low, the negative electrode current collector cannot follow the volume change of the negative electrode mixture, and expands and causes deformation including wrinkles and breaks. If the negative electrode current collector expands and causes deformation such as wrinkles, short circuit causes between a positive electrode and a negative electrode, or the gap between the positive electrode and the negative electrode changes, to make a uniform electrode reaction hard and may result reduced charging/discharging cycle durability. If the negative electrode current collector breaks, the capacity per unit volume reduces and results decreased battery performance of the lithium ion secondary battery. So, if a copper foil is used as the negative electrode current collector, the copper foil is demanded a high tensile strength.
By the way, in the manufacturing process of a negative electrode material, high-temperature is loaded when a negative electrode mixture layer is provided on the surface of a negative electrode current collector. In the popular copper foils, tensile strength decreases if a high-temperature is loaded. So, copper foils used as a negative electrode current collectors are demanded to maintain a high tensile strength also after the heat treatment. For example, Patent Document 2 discloses an electro-deposited copper foil provided with a hard nickel-plated layer as such a copper foil. Patent Document 3 discloses an electro-deposited copper foil provided with a cobalt-plated layer or a cobalt-nickel alloy-plated layer. Patent Documents 2 and 3 disclose that these electro-deposited copper foils maintain a sufficient-level tensile strength even after heat treatment.